A trial to determine the effectiveness of hydroxyurea in Sickle Cell Anemia. Sickle cell disease is a complex syndrome with multiple organ system disturbances brought about through the interplay of genetic, humoral, vascular, and environmental factors. The course of the disease can be one of abrupt and gradual attacks and remissions, thus impairing function, permanently damaging organs, and ultimately causing death. The objectives of the study will be to determine whether there are any long-term adverse effects of treatment of sickle cell anemia with hydroxyurea. The specific objectives in the long-term follow-up of the anemia patient population will be: to identify newly arising, serious medical conditions and progressive impairment of failure of specific organs or systems; to assess all cause mortality and classify causes of death; to classify birth defects occurring in the offspring of patients since enrollment in the study; to assess quality of life for the patients; and to compare the function of specific organs or symptoms, all cause and cause specific mortality, frequency of birth defects and quality of life to those observed in relevant defined patient populations such as the Clinical Study of Sickle Cell Disease. Patients are eligible to enter this study if they have had a history of enrollment in a hydroxyurea study. Approximately 267 patients will be enrolled. They will have completed five annual visits to a clinic. They will be taking medication according to the direction of their own physicians and will know what the medications are. They will help keep records of their medications. They will visit the research center once a year for four years to see a research doctor. At each visit, a blood sample will be drawn to find out about the general health of the subject and to study the blood cells. Part of the sample will be used on that day and part will be frozen for further use. A part of the blood taken will be used to look for changes in the genes in the cells.